The present invention generally relates to a wireless communication technique, and more particularly to a wireless communications system, a base station, and a mobile station that realize orthogonalization of uplink paths with the maximum receive-power among multiple users.
Succeeding the International Mobile Telecommunication 2000 (IMT-2000), which is the global standard for the third generation wireless communications, the fourth generation wireless communications system is being developed. For example, a technique is proposed to flexibly and totally support systems throughout the environment, covering the multi-cell environment for cellular systems and the isolated cell environment including hot spot areas and indoor spaces, and to improve the frequency use efficiency under both types of cell environment. See N. Maeda, Y. Kishiyama, K. Higuchi, H. Atarashi, and M. Sawahashi, “Experimental Evaluation of Throughput Performance in Broadband Packet Wireless Access Based on VSF-OFCDM and VSF-CDMA”, IEEE PIMRC, September 2003.
In this publication, direct sequence-code division multiple access (DS-CDMA) is discussed as the major candidate for the wireless access protocol applied to the reverse link from a mobile station to a base station in the fourth generation wireless communications system.
However, when applying the DS-CDMA to the uplink or the reverse link, the adverse affect of multiple access interference (MAI) increases. Multiple access interference is mutual interference between signals from mobile stations due to different propagation conditions (such as transmission delay time or link fluctuation). To reduce multiple access interference, a transmission timing control technique is proposed and discussed in Een-Kee Hong, Seugn-Hoon Hwang and Keum-Chang Whang, “Synchronous Transmission Technique for the Reverse Link in DS-CDMA Terrestrial Mobile Systems,” pp. 1632-1635, Vol. 46, No. 11, IEEE Trans. on Commun., November, 1999. In this transmission timing control, orthogonal spreading codes are assigned to the respective accessing users using a common scramble code among cells, and transmission timing is controlled such that the receive timings of signals from the accessing users agree with each other at the base station.
Multiple access interference among simultaneously accessing users can be reduced by bringing the receive timing of signals from the accessing users in agreement with each other at the base station, and the frequency use efficiency can be improved by using orthogonal spreading codes.
On the other hand, the Third Generation Partnership Project (3GPP), which is a collaboration project for developing technical specifications for IMT-2000 wireless access protocols and backbone networks, proposes a transmission timing control technique for DS-CDMA wireless access in the circuit switching mode mainly for low-rate voice communication channels. See “Study Report for Uplink Synchronous Transmission Scheme (USTS)”, 3GPP TR 25.854.
With this proposal, to realize orthogonalization among user signals, 10-milisecond-length random access channels of the respective users are used for the first synchronization, and transmission delay times are measured in the continuously transmitted data channels to complement synchronization among users.
However, it is expected that the main stream of wireless signal transmission is shifting from continuous transmission for voice communications to burst packet data transmission.
The transmission timing control discussed in the latter publication is on the basis of the circuit-switching mode for low-rate speech channels. In reality, it is difficult to bring the receive timings of a number of low-rate physical channels in a moment. Because of a large number of physical channels, it is hardly expected that multiple access interference can be reduced effectively through orthogonalizaiton of the respective physical channels.
In addition, due to the large number of simultaneously accessing physical channels, processing workload increases, and the throughput and/or the process efficiency cannot be improved.
In contrast, in the packet transmission in wireless sections, the number of simultaneously accessing physical channels in a time slot is less, as compared with the former case. Consequently, it is expected that multiple access interference can be reduced effectively through orthogonalization of simultaneously accessing user signals by controlling transmission timings.